The tong-term objective of this project is to identify the neurophysiological processes underlying speech perception. A defining characteristic of speech sounds is that they are perceived categorically. That is, different acoustic stimuli tend to be grouped into perceptual categories, resulting in better discrimination across than within category boundaries. Our main hypothesis is that the degree of left hemispheric dominance for processing of speech sounds is related to their categorical perception rather than to any special status as speech sounds. We hypothesize that the left hemisphere is specialized for categorical perception of sounds, both speech and nonspeech, whereas the right hemisphere is specialized for continuous discrimination of sounds. Functional magnetic resonance imaging (fMRI) and event-related potential (ERP) techniques will be used to achieve five specific aims: (1) Identify brain areas underlying categorical and continuous perception of speech and speech-like sounds. We predict that categorical perception will be associated with neural activation in regions of the left auditory cortex not activated during perception of acoustically matched but continuously-perceived sounds. (2) Define the relation between categorical perception of sounds and associated patterns of hemispheric lateralization. We predict that categorical perception will result in greater leftward lateralization of neural activation than continuous perception of acoustically matched sounds. (3) Define the neural processes underlying plasticity of the brain in shifting from categorical to continuous perception and from continuous to categorical perception, through training. We predict that the learning and unlearning of perceptual categories will induce hemispheric shifts in processing and associated activation of neural substrates for category representations. (4) Test the effects of attention on categorical perception by comparing brain activation patterns during oddball discrimination under pre-attentive and under active target detection conditions. (5) Explore the temporal course of activation of brain areas involved in categorical and continuous perception, based on fMRI-constrained dipole modeling of the neural generators underlying each type of perception. Speech perception plays a fundamental role in human cognition. Understanding the neurophysiological basis of categorical perception of speech and its relation to hemispheric lateralization may suggest means of improving the treatment of disorders such as dyslexia, specific language impairment and schizophrenia, and means of improving rehabilitation methods in diseases affecting the temporal lobe unilaterally, such as stroke and temporal lobe epilepsy. [unreadable] [unreadable]